I think it's mostly that if I take a shot, then tilt the camera down, then bring it back up and take a shot without thinking, the focal length can change on my 24-70mm. I like to fidget and I often knock the zoom or it slides on it's own between shots. So it's a point for me to pay attention to.
....

You've got zoom drift. None of my zooms do that, thank goodness. I saw a band that you put around your lens to combat drift, but I forget where I saw it. It seemed like a simple solution to what, I'd guess, would be an aggravating problem.